Ethylene copolymers have heretofore been molded by various molding methods, and used in many fields. The requirement for the characteristics of the ethylene copolymers differs depending on the molding methods and uses. For example, when an inflation film is molded at a high speed, it is necessary to select an ethylene copolymer having a high melt tension compared with its molecular weight in order to stably conduct high speed molding without fluctuation or tearing of bubbles. An ethylene copolymer is required to have similar characteristics in order to prevent sag or tearing in blow molding, or to suppress width shortage to the minimum range in T-die molding.
Further, in extrusion molding, it is important to have an excellent flowability under high shearing during extrusion in order to improve quality of molded article and reduce electric power consumption.
A high-pressure low density polyethylene has a high melt tension compared with an ethylene copolymer prepared with a Ziegler type catalyst, and is used as a material for films and hollow containers. The high-pressure low density polyethylene as described above has low mechanical strength such as tensile strength, tear strength and impact strength, and in addition it has also low heat resistance, low stress cracking resistance, etc.
Of the ethylene polymers obtained during using Ziegler type catalysts, an ethylene polymer obtained by chromium type catalyst is relatively excellent in melt tension but inferior in heat stability.
On the other hand, Japanese Patent L-O-P Nos. 90810/1981 and 106806/1985 propose a method for improving the melt tension and blow ratio (die/swell ratio) of ethylene polymers obtained by using Ziegler type catalysts, especially a titanium type catalyst.
The ethylene polymers obtained by using a titanium catalyst, however, especially the low density ethylene polymers generally have problems such as their broad composition distribution and stickiness of their molded articles such as films.
Accordingly, the advent of ethylene polymers having an excellent melt tension, a high flowability, a narrow composition distribution, and an excellent heat stability will industrially be of great value.
There has recently been developed a new Ziegler type catalyst for olefin polymerization comprising a zirconium compound and an aluminoxane, said catalyst being capable of producing ethylene/.alpha.-olefin copolymers with high polymerization activities. There has also been proposed a process for the preparation of ethylene/.alpha.-olefin copolymers using such a new type catalyst.
For example, Japanese Patent L-O-P No. 19309/1983 discloses a process for polymerizing ethylene with one or at least two C.sub.3 -C.sub.12 .alpha.-olefins at a temperature of -50.degree. to 200.degree. C. in the presence of a catalyst composed of a transition metal compound represented by the formula EQU (cyclopentadienyl).sub.2 Me R Hal
wherein R is cyclopentadienyl, C.sub.1 -C.sub.6 alkyl or halogen, Me is a transition metal and Hal is halogen, and a linear aluminoxane represented by the formula EQU Al.sub.2 OR.sub.4 [Al(R)--O].sub.n
wherein R is methyl or ethyl, and n is a number of 4 to 20, or a cyclic aluminoxane represented by the formula ##STR1## wherein R and n are as defined above. Japanese Patent L-O-P No. 19309/1983 discloses an invention relating to processes for preparing a linear aluminoxane represented by the formula ##STR2## wherein n is a number of 2 to 40, and R is C.sub.1 -C.sub.8 alkyl, and a cyclic aluminoxane represented by the formula ##STR3## wherein n and R are as defined above. The same Patent Publication also discloses a process for the polymerization of olefin using a catalyst prepared by mixing, for example, methylaluminoxane prepared by the above-mentioned process and a bis (cyclopentadienyl) compound of titanium or zirconium. Japanese Patent L-O-P No. 35005/1985 discloses a process for preparing an olefin polymerization catalyst, wherein the process comprises reacting an aluminoxane represented by the formula ##STR4## wherein R.sup.1 is C.sub.1 -C.sub.10 alkyl, and R.sup.0 is R.sup.1 or R.sup.0 represents--O-- by linkage, with a magnesium compound at first, then chlorinating the reaction product, and treating with a compound of Ti, V, Zr or Cr.
Japanese Patent L-O-P No. 35006/1985 discloses a catalyst composed of mono- di- or tricyclopentadienyl-transition metals (a) (transition metals being at least two different metals) or their derivatives and an alumoxane (aluminoxane) in combination. Example 1 of this Patent publication discloses that ethylene and propylene are polymerized to form a polyethylene in the presence of a catalyst composed of bis(pentamethylcyclopentadienyl)zirconiumdimethyl and an aluminoxane. In Example 2 of this Patent publication, ethylene and propylene are polymerized to form a polymer blend of a polyethylene and an ethylene/propylene copolymer in the presence of a catalyst composed of bis(pentamethylcyclopentadienyl)zirconiumdichloride, bis(methylcyclopentadienyl)zirconium dichloride and an alumoxane.
Japanese Patent L-O-P No. 35007/1985 discloses a process wherein ethylene alone is polymerized, or ethylene and an .alpha.-olefin of not less than 3 carbon atoms are copolymerized in the presence of metallocene, and a cyclic aluminoxane represented by the formula ##STR5## wherein R is an alkyl group of 1 to 5 carbon atoms, and n is an integer of 1 to about 20, or a linear aluminoxane represented by the formula ##STR6## wherein R and n are as defined above.
Japanese Patent L-O-P No. 35008/1985 discloses a process for the preparation of a polyethylene or a copolymer of ethylene and a C.sub.3 -C.sub.10 .alpha.-olefin, wherein a catalyst system comprising not less than two types of metallocene and an alumoxane is used.
Though the catalysts formed from a transition metal compound and an aluminoxane proposed by the prior art are excellent in polymerization activities, especially ethylene polymerization activities compared with those catalysts having been known prior to the appearance of these catalysts and formed from a transition metal compound and an organolauminum compound, most of the catalysts are soluble in the reaction system, and in most cases the processes for the preparation are limited to a solution polymerization system. In addition, the catalysts have such a problem that the productivity of a polymer is lowered due to a marked increase in the viscosity of the polymer-containing reaction solution when the manufacture of a polymer having a high molecular weight is tried, that the polymer obtained by after-treatment of polymerization has a low bulk specific gravity, and that the preparation of a sphere polymer having excellent particle properties is difficult.
On the other hand, polymerization of olefin has been tried in a suspension polymerization system or a gas phase polymerization system by using catalysts in which at least one of the transition metal compound component and the aluminoxane component described above is supported on a porous inorganic oxide carrier such as silica, alumina and silica-alumina.
For example, Japanese Patent L-O-P Nos. 35006/1985, 35007/1985 and 35008/1985 described above disclose that there can be used catalysts in which a transition metal compound and an aluminoxane are supported on silica, alumina, silica-alumina, etc.
Furthermore, Japanese Patent L-O-P Nos. 106808/1985 and 106809/1985 disclose a process for the preparation of a composition composed of an ethylene polymer and a filler, which process comprises polymerizing ethylene or copolymerizing ethylene and .alpha.-olefin in the presence of a product prepared by contacting a highly activated catalyst component comprising a hydrocarbon-soluble titanium compound and/or a zirconium compound with a filler, an organoaluminum compound and a filler having an affinity for polyolefin.
Japanese Patent L-O-P No. 31404/1986 discloses a process for polymerizing ethylene or copolymerizing ethylene and an .alpha.-olefin in the presence of a catalyst mixture composed of a transition metal compound and a product obtained by the reaction of trialkylaluminum and water in the presence of silicon dioxide or aluminum oxide.
Furthermore, Japanese Patent L-O-P No. 276805/1986 discloses that olefin is polymerized in the presence of a catalyst composed of a zirconium compound and a reaction mixture obtained by reacting an aluminoxane with trialkylaluminum at first, and further by reacting the resultant reaction mixture with such an inorganic oxide having a hydroxide group on the surface as silica.
Still furthermore, Japanese Patent L-O-P Nos. 108610/1986 and 296008/1986 discloses a process for polymerizing olefin in the presence of a catalyst in which a transition metal compound such as metallocene and an aluminoxane are supported on a carrier such as an inorganic oxide.
However, during the polymerization or copolymerization of olefin in a suspension or gas phase by using such a solid catalyst component supported on a carrier as described in the above-mentioned Patent publns., the catalyst component considerably lowers the polymerization activities compared with the above-described solution polymerization, and the resulting polymers do not have a satisfactory bulk density.